The term “rigging” (sometimes referred to as industrial rigging or field rigging) is the branch of securing heavy loads in order to prepare the load to be lifted, moved or transported. Rigging usually refers to the ropes, wires, slings, and chains used to secure the load and not the cranes, boomlifts, air skates, forklifts, or other powered equipment that provides the actual force/energy to lift the object.
Wire rope slings made of a plurality of metal strands twisted together and secured by large metal sleeves or collars are common in the industry. Since wire rope slings are made of metal, they do not require any protection that may be afforded by a covering material. During the past thirty years, industrial metal slings have seen improvements in flexibility and strength. However, compared to non-metal or synthetic fiber slings, metal slings are relatively stiff and inflexible.
Synthetic fiber slings have gained popularity over the last approximately twenty years and are replacing metal slings in many circumstances. Thousands of synthetic slings are being used on a daily basis in a broad variety of heavy load lifting applications which range from ordinary construction (e.g., nuclear power plants, skyscrapers and bridges), plant and equipment operations, to ship building (e.g., oil rigs), and the like.
An advantage of synthetic slings over metal slings is that they have a very high load-lifting performance strength-to-weight ratio which provides for a lighter, more flexible and even stronger slings than their heavier and bulkier metal counterparts. An important disadvantage is that synthetic slings require extra steps (primarily encasing the lifting core inside a protective cover), in its manufacturing process.
Synthetic slings are usually comprised of a lifting core made of twisted strands of synthetic fiber and an outer cover that protects the core. The most popular design of synthetic slings is a roundsling in which the lifting core forms a continuous loop and the sling is generally ring-shaped in appearance. The lifting core fibers of such roundslings may be derived from natural materials (e.g., cotton, linen, hemp, etc.), but are preferably made of hemp, linen, etc. synthetic materials, such as polyester, polyethylene, nylon, and the like. The outer covers of synthetic slings are preferably made of synthetic materials and are designed to protect the core fibers from abrasion, cutting by sharp edges, or degradation from exposure to heat, cold, ultraviolet rays, corrosive chemicals or gaseous materials, or other environmental pollutants.
A popular method of manufacturing of prior art roundslings is to twist a plurality of yarns together to form a single strand; the strand was then rolled into an endless parallel loop that formed the core. In a separate step, the cover would be manufactured as a flat piece; then the lifting core would be laid on the flat material, and the flat piece of cover material would be bent around the endless core; finally, the edges of the cover are sewn together thereby encasing the core. This method of manufacturing roundslings is time consuming and labor intensive thus increasing the costs to manufacture the sling.
An important advancement in the rigging industry was the invention of multiple-path slings by Dennis St. Germain. (See U.S. Pat. No. 4,850,629, titled Multiple Path Sling Construction). The manufacturing process for a two-core roundsling is more difficult since it requires more time and labor than a single-core roundsling.
Machines used to manufacture round slings and multiple-path slings are still relatively labor intensive. Accordingly, there is a need in the industry to reduce the amount of labor needed in the manufacturing of synthetic slings.